Distance between two speakers

Two in-phase loudspeakers are 2.35 m apart. They emit sound with a frequency of 425.0 Hz. A microphone is placed half-way between the speakers and then moved along the line joining the two speakers until the first point of constructive interference is found. At what distance from that midpoint is that first point? The speed of sound in air is 342 m/s.

2. Relevant equations

w=wavelength
w=v/f

3. The attempt at a solution

I used the above equation to find the wavelength (0.805m), divided the distance between the speakers by 2 (2.35/2= mid point) and then subtracted one wavelength from this to get what I thought would be the answer (0.370m). But it is not correct and I am really lost. I know there is another old thread like this but that person did the same as me and never got the right answer.

Consider that since the sound arriving at the midpoint at the same time, shouldn't a half λ to each side experience constructive interference, because the wave at that point will be delayed from arriving from one speaker by 1/2λ and from the other speaker will require another 1/2λ longer?

At the midpoint, the distance from speaker 1 is the same as the distance from speaker 2 - the difference between the distances is 0. Constructive interferance occurs when the difference is 0, λ, 2λ, 3λ etc., so a point before the midpoint would be where the wave from speaker 2 has traveled one wavelenght longer than the wave from speaker one, right?